(i) Field of the Invention
The present invention relates to a USB storage device for storing information using a semiconductor memory.
(ii) Background Art
These days, attachable and detachable USB storage devices, each of which comprises an installed flash memory and an interface according to the USB (Universal Serial Bus) standard and has a compact configuration by integrating its USB connector portion into the main body, are widely known as described in Publication of Unexamined Japanese Patent Application No. 2000-259275. Such USB storage devices with a compact configuration can be manufactured at a low cost, and are utilized for companies' sales promotion items into which software of commercials and others is written.
Most of these USB storage devices are provided with a physical switch, such as a write protect switch, to prevent the software from being deleted by mistake. By changing the switch, switching between the state in which reading, writing and deleting is possible and the state in which only reading is possible is available. In some cases an inherently unwritable memory, such as a ROM, is used to completely prevent writing and deleting
However, when a ROM or the like is used in a USB storage device to prevent a user from writing information, the USB storage device has only limited applications and thus reduced usability. In other cases, it is impossible to write temporary files or data during execution of the software, which limits the function of the software itself. When a write protect switch as mentioned above is used, the write protect switch is sometimes turned off due to some physical factor unknown to the user, and thereby unintended writing or deleting may be performed.
To solve these problems, technology as described in Publication of Unexamined Japanese Patent Application No. 10-289159 has been devised. According to the technology, a portable-type writable medium has a hybrid configuration by virtually providing an unwritable area and an unreadable area.
When applying the above described technology to a USB storage device, practical working methods may be as the following.
In one working method, a USB storage device is provided with two internal USB storage device units (deleting and writing is restricted in one of the USB storage device units) and a hub connecting these USB storage device units. However, there is a problem that such a USB storage device has a complex configuration as it actually has two USB storage device units therein. There is another problem that when the USB storage device is connected to a host, connecting processing takes time since the host first recognizes the USB storage device as a HUB device and then initializes the same number of mass storage class drivers as the number of internally provided USB storage device units.
In another working method, the number of endpoints in the USB standard is increased. In this case, however, a controller provided in the USB storage device is required to have the same number of FIFO buffers as the number of the endpoints. Although there is a newer controller without multiple FIFO buffers by the technology of using virtual endpoints, all these controllers have complex configurations, and thus lead to an increased manufacturing cost.
An object of the present invention, which has been made to solve the above problems, is to provide a USB storage device which has as simple a structure as possible and achieves the function of preventing stored software from being deleted by mistake.